Conventionally, spark ignition of an internal combustion engine is controlled by mechanical contact breaker points which are operated by a rotary cam driven from the crankshaft of the engine. Timing of the spark ignition is controlled by moving the angular position of the contact breaker points relative to the cam's axis of rotation in dependence upon the level of partial vacuum obtaining in the inlet manifold to the engine.
Recently, electronic ignition systems for internal combustion engines have been developed. The electronic systems permit the timing of the spark ignition to be controlled in dependence not only upon the aforementioned vacuum level but also in dependence upon a plurality of other engine operating parameters and consequently permit the engine to operate more efficiently. The electronic ignition systems do not require the conventional contact breaker and cam arrangement, but some means is required to provide to the system an electrical signal indicative of the angular position of rotation of the engine in order that the system can control the timing of the spark ignition. Moreover, the angular position of rotation of the engine crankshaft needs to be monitored much more accurately than is possible with the conventional cam and contact breaker arrangement if the advantages of efficiency of engine operation made possible by the electronic ignition systems are to be maximized.
One prior art proposal for monitoring engine rotation is to fit to the flywheel of the engine, a series of permanent magnets accurately spaced apart around the periphery of the flywheel. The magnets are fitted in holes drilled in the flywheel. A pickup coil is mounted on the engine close to the flywheel such that upon rotation thereof each magnet induces an electrical pulse in the coil as the magnet passes the coil. Each pulse is thus indicative of the occurrence of a particular angular position of the engine. It is, however, difficult with this arrangement to achieve sharp pulses, as is required for an electronic ignition system, which indicate accurately when each of the magnets moves into alignment with the coil. This difficulty is due to the fact that, as a magnet approaches and passes the coil, a relatively long rise and fall of the induced pulse occurs, so that it is difficult to determine the peak of the pulse and hence the instance of alignment of the coil and magnet. In order to reduce but not overcome this difficulty, the coil is mounted as close as possible to the flywheel, typically less than 0.1 inch, which is difficult to achieve in practice without adding to the cost of the engine. It will also be appreciated that the required accurate drilling of the flywheel to fit the magnets adds significantly to the cost of the engine. Another difficulty with this prior art proposal is that in use thereof dirt tends to build up on the pickup coil, which causes the peak amplitude of the induced pulses to reduce with time as the dirt builds up, thereby making it difficult to use threshold circuits to improve the shape of the induced pulses. If a threshold circuit is used with a fixed threshold, the amplitude of the threshold must be relatively low in order to accomodate the reduction of pulse amplitude that will occur with time for the magnet induced pulses if the induced pulses are always to exceed the threshold. The relatively low thresehold accordingly provides for inaccurate results.